The need for scrambling video signals is well known in the art. Various methods of scrambling signals are known. One method of scrambling involves inverting a signal or a portion of a signal at the transmitter side and reinverting the signal or portion at the receiver side to reconstruct the proper video signal.
At the transmitter side, the portion of the video signal that is inverted is inverted about a selected axis. Typically, this axis may be a function of system parameters. For example, if +100 IRE represents the peak white level of a video signal and -40 IRE represents the sync tip level or the most negative portion of the signal, then a likely choice for the axis of inversion would be +30 IRE. This point is half way between the most negative and most positive portions of the signal. Therefore if the most negative portion of the signal is inverted about this axis it should not exceed the most positive portion (+100 IRE). Similarly, if the white peak level is inverted about this axis, it will not fall below the most negative portion (-40 IRE).
For a better understanding of video signals and some inversion schemes, attention is now drawn to FIGS. 1A-1D. FIG. 1A illustrates a sketch of a video signal. This figure and the labeled parts thereof is intended to serve as a model to assist the reader in being able to identify some key portions of the video signals of this and other figures described in the specification. For clarity these portions are not generally labeled in the other figures. Those skilled in the art will recognize the generality of the figures.
With reference to FIG. 1A, it can be seen that each line of video signals is characterized by a horizontal sync pulse, 101, represented by the sync tip pulse or the most negative state of the video signal. This pulse normally last 4.7 microseconds and is repeated 15,734 times each second in the NTSC television system used in the U.S. and many other countries. Following the sync pulse, the signal voltage returns to the blanking level or black level, which is used as a reference level. By common convention, the amplitude of the blanking level is considered to be 0 IRE, a unit of measure adopted by the Institute of Radio Engineers (now IEEE). The sync tip is at a level of -40 IRE, while peak white is at a level of +100 IRE. This maximum normal excursion of the TV signal of 140 IRE is conventionally equated to 1 volt peak to peak, though other voltage levels are sometimes employed internal to a particular piece of equipment. Following the return to a blanking level after a sync tip portion, and after a delay known as the breezeway, 102, the color burst, 103, occurs. This color burst is composed of eight cycles (nominally) of the color subcarrier, 3.58 MHz in NTSC transmission. The amplitude and, more importantly, the phase of the color burst are essential to proper recovery of the color information, as is well understood by those skilled in the art. After a delay following the color burst, the active video interval, indicated generally as 107, begins. The time from the end of the sync pulse to the beginning of active video is called the back porch, 106. The end of the active video defines the front porch, 104, shown twice to emphasize that the signal repeats. The entire interval from the beginning of the front porch to the end of the back porch is collectively known as the horizontal blanking interval (HBI), 105.
The active video interval, 107, actually consists of various voltages representing the brightness (luminance) of the image, plus a color subcarrier (not shown), which carries color saturation ("purity") information as amplitude modulation, and color value ("tint") represented by it's phase with respect to burst 103. This pattern of sync and active video is repeated for 252.5 lines (including vertical blanking). This number of lines constitutes one "field" and is followed by an interleaved second field. The two fields together make up a "frame," or one complete picture. Typically, the frame rate is 30/sec, that is, frames are produced 30 times each second.
FIGS. 1B-1D illustrates some of the various ways in which a video signal may be inverted. In FIG. 1B the active video line is inverted but the horizontal blanking interval (HBI) is not. This is known by those skilled in the art as inverted video with non-inverted sync. FIG. 1C illustrates an inverted HBI with normal active video. FIG. 1D illustrates a combination of the two, that is, inverted video and inverted HBI.
In the case of active video inversion only, FIG. 1B, the sync occurs normally, however the synchronization recovery circuits in the TV receiver have a hard time recovering the sync. This is because the sync circuits are designed to look for the most negative portion of the video signal. As shown, in the case of inverted video, the peak white signal has now been moved to the amplitude of the sync, so the sync circuits will not be able to distinguish sync from peak white. In the event the sync circuits do successfully identify the sync information (for example, in a dark scene having no peak white), the picture will appear as the negative of the real picture, because the light and dark levels have been reversed. Further, the color information will be rendered incorrect because the phase of the color subcarrier is reversed in the inversion process.
FIG. 1C illustrates inverted sync with noninverted video. In this case the picture information would appear correct if sync recovery was possible, but since the sync has been inverted, the sync circuits will not be able to identify the sync, rendering the picture garbled.
FIG. 1D combines the above inversion methods by inverting both the sync and the video.
A desirable method of video scrambling would be to allow changing from one of these modes to another either randomly or based upon some predetermined condition such as average picture level (APL). These modes have been used singly or in combination in scrambling systems employed during the last few years. Other modes of scrambling are known and may also be used with the present invention. These other known modes have not been shown, for the sake of clarity. Some other known modes of scrambling include sync suppression, dynamic sync suppression, drop field and various combinations of the above.
Past systems have suffered from at least two problems which create artifacts in the recovered signal. If sync is inverted, the automatic gain control (AGC) circuits of the demodulator, employed to recover the signal before descrambling, will not be able to recover a good gain reference for automatic gain control (AGC). This is so because the inverted sync pulse is normally transmitted as the highest amplitude of the modulated signal, which the AGC circuits look for in order to normalize the amplitude of the received signal. But when the sync pulse is inverted, the peak value of the modulated signal corresponds to the peak white level in the scene. Since the peak white level is a function of the picture, it does not form a satisfactory reference. One possible solution to this problem would be to detect the minimum value of the carrier and use it as a reference as is done in the SECAM television system employed in France and other countries. However, since the same demodulator circuit is called upon to handle both scrambled and nonscrambled signals (inverted and noninverted), it would therefore be necessary to provide two AGC detectors for recovering scrambled and unscrambled signals, one for detecting inverted and one for detecting non-inverted signals. This would significantly add to cost of the circuit. Moreover, the difficulty of matching the performance of two difference AGC detectors is formidable.
A second important deficiency of past systems arises due to the "calculation" of the axis of inversion, shown generally as 108 in FIG. 1A. This axis is the voltage level (measured in IRE units) about which a portion of the signal to be inverted is rotated. One can imagine that the signal is "anchored" to the axis of rotation and flips about it so that its negative peak becomes its positive peak and vice versa. In order to effect proper descrambling, the axis of inversion in the scrambler and the descrambler must be identical since if the signal or a portion thereof is inverted about an axis in the scrambler, it must be reinverted about the same axis in the descrambler to accurately reproduce the correct picture signal information. Should the descrambler have a different axis of inversion than the scrambler, the recovered signal will be shifted with respect to the transmitted signal. This results in either stretched or compressed video signals and/or sync pulses. In either case the brightness levels of the recovered video will be altered, leaving an undesirable artifact in the picture. This artifact is especially troublesome if the method of video inversion is changed frequently (a desirable condition for immunity to pirating).
Past systems have relied on factory calibration to maintain the integrity of the axis of inversion in a system. This scheme is unsatisfactory. For example, some systems may measure the level of the sync signal, and "count off" the distance to the axis of inversion. As know to those of skill in the art, a particularly convenient axis of inversion is +30 IRE, half way between the sync tip (-40 IRE) and peak white (+100 IRE). If the normal amplitude of the recovered signal at the descrambler is 1 volt, then the voltage difference between the sync and the axis is EQU (70/140).times.(1 volt)=0.5 volt
But suppose that the depth of modulation at the modulator is changed such that the recovered signal is not 1 volt, but is 0.9 volts instead. The descrambler has no knowledge of this change, so it will assume the axis to be 0.5 volt from the sync tip. However, the axis is really EQU (70/140).times.(0.9 volt)=0.45 volt
from the sync tip. The descrambler places the axis in error by 0.05 volt, which can be shown equivalent to an error of 7.78 IRE. When the axis of inversion is in error, it can be shown that the error in the resultant video is twice the error in the axis, or 15.56 IRE.
One of ordinary skill in the art will further appreciate that other conditions (e.g., drift) in the transmitter or receiver can cause errors in the calculation of the axis of inversion.
A further deficiency of past systems is manifested in the headend of the CATV system, where the signal is scrambled. The modulator, which must impress the scrambled signal on a RF carrier, includes a sync tip clamping circuit used to normalize the peak envelope of the modulated signal to a desired level. Reference to FIG. 1C or D shows that in these scrambling modes, the sync does not occur at the expected peak negative of the video signal, rendering the clamp useless. Past systems have required that a modified modulator be used to retain the clamp function. This modification typically was rather extensive and rendered the modulator on which it was performed, void of the manufacturer's warrantee.